1. Field of the Invention
This invention relates to a process and apparatus for regenerating fluidized cracking catalyst. More particularly, it relates to a process and apparatus including staged regeneration and separation of flue gas from catalyst particles to minimize--or substantially eliminate--hydrothermal deactivation and NO.sub.x formation.
2. Discussion of the Prior Art
The field of catalytic cracking, and particularly fluid catalyst operations, has undergone significant development and improvements due primarily to advances in catalyst technology and product distribution obtained therefrom. With the advent of high activity catalysts, and particularly crystalline zeolite cracking catalysts, new areas of operating technology have been encountered, requiring even further refinements in processing techniques to take advantage of the high catalyst activity, selectivity and operating sensitivity.
By way of background, the hydrocarbon conversion catalyst usually employed in a fluid catalytic cracking (FCC) installation is preferably a high activity crystalline zeolite catalyst of a fluidizable particle size. The catalyst is transferred in suspended or dispersed phase condition generally upwardly through one or more riser conversion zones (fluid catalytic cracking zones), providing a hydrocarbon residence time in each conversion zone in the range of 0.5 to about 10 seconds, and usually less than about 8 seconds. High temperature riser hydrocarbon conversions, occurring at temperatures of at least 1000.degree. F. or higher and at 0.5 to 4 seconds hydrocarbon residence time in contact with the catalyst in the riser, are desirable for some operations before initiating separation of vaporous hydrocarbon product materials from the catalyst. Rapid separation of catalyst from hydrocarbons discharged from a riser conversion zone is particularly desirable for restricting hydrocarbon conversion time.
During the hydrocarbon conversion step, carbonaceous deposits accumulate on the catalyst particles and the particles entrain hydrocarbon vapors upon removal from the hydrocarbon conversion step. The entrained hydrocarbons are subjected to further contact with the catalyst until they are removed from the catalyst by stripping gas in a separate catalyst stripping zone. Hydrocarbon conversion products separated from the catalyst and stripped materials are combined and typically passed to a product fractionation step. Stripped catalyst (spent catalyst) containing deactivating amounts of carbonaceous material, hereinafter referred to as coke, is then passed to a catalyst regeneration operation.
In catalyst regeneration, the spent catalyst contacts oxygen to burn off coke. However, spent catalyst contains hydrogen-containing components, such as coke, adhering thereto. This causes hydrothermal degradation because the hydrogen reacts with oxygen in the regenerator to form water.
U.S. Pat. No. 4,336,160 to Dean et al attempts to reduce hydrothermal degradation by staged regeneration. However, the first stage of the regeneration process of Dean et al employs a dense bed which provides an opportunity for hydrothermal deactivation.
A major trend in fluid catalytic cracking processing has been modifications to the process to permit it to accommodate a wider variety of feedstocks, in particular, stocks that contain more nitrogen than had previously been permitted in a feed to a fluid catalytic cracking unit.
Along with the development of process modifications and catalysts which could accommodate heavier, dirtier feeds, there has been a growing concern about the amount of nitrogen contained in the feed that ended up as NO.sub.x in the regenerator flue gas. Some attempts have been made to minimize the amount of NO.sub.x discharged to the atmosphere through the flue gas by employing multiple beds in a fluid catalytic cracking regenerator.
U.S. Pat. No. 4,325,833 to Scott discloses a three-stage regenerator directed to NO.sub.x removal. Scott discloses that his middle stage contains a substantially oxygen-free atmosphere to convert NO.sub.x to N.sub.2. However, flue gas from lower beds contact with catalyst from upper beds. This is detrimental because the flue gas contains water which can deactivate the catalyst by hydrothermal degradation.
It would be advantageous to provide a process which both minimizes NO.sub.x and hydrothermal degradation.